Fitts’ Law

Another ongoing theme of this blog is the topic of human machine interaction.  Having come from a background of probably the most profound form of human machine interaction on this planet today, flying high performance aircraft off the deck of an aircraft carrier, I have a very passionate point of view on this topic.  The capabilities of today’s emergent interfaces when combined with ever increasing computing power, arguably put us at an inflection point in the history of human kind.

photo credit: Impala74

The cockpit of the modern high performance aircraft is optimized for man-machine interaction.  The placement of every control and instrument is calculated based on a basic set of variables available to the designer:

• “MT” or “T”, the Movement Time critical for the pilot to complete the action.
• “a”, the pilot’s reaction time.
• “b” the speed at which the pilot can complete the movement or intake of the instrument.
• “A” or “D” is the amplitude of the movement or the distance at which the control or instrument is from the pilot’s position.
• “W” is the width of the target measured along the axis of motion. W can also be thought of as the allowed error tolerance in the final position, since the final point of the motion must fall within ±W⁄2 of the target’s center.

All these variables come together in an equation that describes the pilot’s ability to carry out the action¹:

Fitts’ Law is one of the governing equations of human machine interaction and many things can be learned through various derivations of this equation.  One can think about the effects the ever increasing human ability to process information at faster speeds:

• How much is “D” or “W” able to decrease given b going higher?
• If reaction time “a” decreases how does that change the size requirements of the target area or amplitude of the movement?
• How many movements can take place given a richer environment with more complexity if the average human is able to process more?

I often argue that this is the age of amplification and acceleration of human ability.  Whether it be the advancement of our interactions with computers or even the platform of social media as a soap box, we have more ability to do profound things today than ever before.  Experience designers must consider this increase in capability and Fitts’ law is a simple way to think through the potential.

Tagged as: Experience Design, Fitts, Fitts' Law, HCI, human machine interaction, User Experience